The invention relates to lasers and systems for converting laser radiation from one frequency to another, e.g., to generate ultraviolet radiation.
There have been significant advances recently in the development of diode-pumped solid-state lasers. Such lasers are compact, reliable, and easy to use and maintain. Typically, these lasers can be connected to conventional AC outlets and produce output powers of a few watts without the need for cumbersome power supplies and coolant systems.
Most diode-pumped solid-state lasers operate multimode. In a multimode laser, multiple longitudinal modes resonate simultaneously without any defined phase coherence, i.e., phase relationships between the longitudinal modes are allowed to fluctuate randomly. Multimode lasers are different from mode-locked and single-mode lasers.
For example, a mode-locked laser includes an element, e.g., an acousto-optic modulator, an electro-optic modulator, or a Kerr-lens aperture, which prevents longitudinal modes from resonating unless they are in-phase and "locked" with one another. The locked longitudinal modes produce as the laser output a train of short pulses spaced from one another by the round-trip time of the laser cavity and each having a duration proportional to the inverse of the frequency bandwidth of the locked longitudinal modes. In a single-mode laser, a single longitudinal mode is preferentially amplified, thereby producing a laser output having a narrow frequency band and a long coherence length. For a general reference on multimode, mode-locked, and single-mode lasers, see, e.g., A. E. Siegman, Lasers, (University Science Books, Mill Valley, Calif., 1986).
For many applications, e.g., material processing, laser marking, and photolithography, the pulsed output of mode-locked lasers and the narrow-frequency output of single-mode lasers are unnecessary. For such applications, multimode lasers, and in particular multimode diode-pumped solid-state lasers, can be more suitable since they tend to be simpler, more compact, and more efficient than comparable mode-locked and single-mode lasers.
However, for many applications, ultraviolet wavelengths are necessary. For example, in photolithography applications, 300 mJ at 193 nm is typically required to process an eight inch diameter wafer in 300 msec. To achieve such wavelengths, the visible to near infrared output of most solid-state lasers can be passed through non-linear optical (NLO) crystals that convert the infrared output into its harmonics in the ultraviolet region. Unfortunately, such conversions typically have poor efficiencies and often require tight focusing that can damage the NLO crystals and affect stability.